In the field of light detection in particular for a computer mouse with optical sensing device, it is actually necessary to improve the processing of the motion detection of said computer mouse on a work surface. The light reflected on the work surface has to be picked up by a pixel or photo-detector array in order to be processed in a motion detection processing circuit. A comparator array is placed between the pixel or photo-detector array and the motion detection processing circuit. Said comparator array is composed of several comparator units to compare successively several signals from the pixel or photo-detector array. With this motion detection processing circuit of the computer mouse, the varying intensity pattern of a portion of an illuminated work surface can be measured, and the information about the relative motion between the pixel or photo-detector array and the illuminated portion of the work surface can be extracted.
In a conventional computer mouse with detection of light reflected on a work surface, it can be envisaged to adapt an algorithm, which requires both hysteresis and tri-state detection levels. A window comparator circuit is used for tri-state detection levels. It is to be noted that a tri-state refers to three output states, for example one output state below a lower window voltage, one output state inside some window voltage limits, and one output state above an upper window voltage. For that it is generally not possible to conceive said window comparator circuit only with a simple comparator as usually in the light detection devices of the prior art. Furthermore due to the number of signals to be compared in the comparator array, it is not feasible to implement 2-bit A/D converters or two simple comparators, in order to perform a window function for a window comparator circuit. Additionally hysteresis is also required to prevent chatter at the boundaries.
We can cite the JP patent application No. 06-120784 A, which describes a window comparator circuit. Said window comparator circuit is composed of two comparators at the output of said circuit. Said window comparator circuit allows equalizing hysteresis characteristics of a dead zone width in the upper end and the lower end of said dead zone. For that it is used an operational amplifier combined with a voltage setting unit, a reference voltage source and a power source. Two signals from the voltage setting unit are respectively compared to a reference voltage in the two corresponding comparators. Given that two comparators are necessary to perform a comparison, said window comparator circuit is made with too many electronic components, which is a drawback. Furthermore it is difficult to match accurately the two comparators to well define a tri-state comparison function, which is another drawback.